1. Field of the Invention
The present invention relates to a method of generating multi-channel local oscillation frequencies in an ultra wide band communication system, and more particularly, to a method of generating multi-channel local oscillation frequencies in an ultra wide band communication system, the method in which interference with communication via another channel by an unnecessary side band frequency, which may be generated when local oscillation frequencies are generated by using a single side band mixer, is prevented.
2. Description of the Related Art
In general, an ultra wide band communication system has a capacity to transmit a large amount of information per unit time with use of a plurality of sub-band frequency bands each having a certain bandwidth when transmitting desired data.
To realize such an ultra wide band communication system, two methods have been proposed currently; multi-band orthogonal frequency division multiplexing (MB-OFDM) including multiple channels of 528 MHz and direct sequence code division multiple access (DS-CDMA), an access scheme based on impulse radio spread spectrums of two bands. The MB-OFDM of multi-band access scheme offers flexible use despite country specific frequency allocation, and thus is actively developed.
In the MB-OFDM, the entire frequency band from 3.1 to 10.6 GHz is divided into five band groups, and each band group includes multiple channels each having a bandwidth of 528 MHz. The three channels of band group 1, which is the lowest frequency band, are compulsory while the rest of the band groups are optional. In a MB-OFDM UWB communication system, since fourteen channels may be formed in the band of 3.1 to 10.6 GHz, one local oscillation frequency per each channel or a total of fourteen local oscillation frequencies are needed.
FIG. 1 illustrates a conventional method of generating multi-channel local oscillation frequencies. The method shown in FIG. 1 is proposed in “Multi-band OFDM physical layer proposal for IEEE 802.15 Task Group 3a,” and is most widely used currently. As shown in FIG. 1, in the conventional method of generating local oscillation frequencies, center frequencies for the respective band groups are generated and each of the center frequencies is down-converted and up-converted by 528 MHz, thereby obtaining local oscillation frequencies for the fourteen channels in the MB-OFDM system. In this case, a signal of 528 MHz is an adjustment frequency for up-converting and down-converting each of the center frequencies of the band groups. FIG. 2 illustrates a conventional apparatus generating local oscillation frequencies.
As shown in FIG. 2, the conventional apparatus, generating the local oscillation frequencies, includes a voltage control oscillator 111 generating a reference frequency (e.g. 6336 MHz); a phase-locked loop (PLL) 112 fixing an output frequency of the voltage control oscillator 111; an auxiliary frequency generator 11 including a plurality of dividers generating a plurality of auxiliary frequencies (e.g. 264 MHz, 528 MHz, 1056 MHz and 2112 MHz) by dividing the reference frequency; a center frequency generator 12 including multiplexers 121a and 121b selectively outputting the reference frequency and the auxiliary frequencies and a plurality of single side band mixers 122a to 122c generating center frequencies of the respective band groups by selectively mixing the reference frequency and the auxiliary frequencies; and a local oscillation frequency generator 13 including single side band mixers 131a and 131b generating multi-channel local oscillation frequencies by up-converting and down-converting the center frequencies by 528 MHz and a multiplexer MUX 132 selectively outputting the multi-channel local oscillation frequencies.
In the conventional apparatus generating multi-channel local oscillation frequencies, a plurality of auxiliary frequencies for generating the multi-channel local oscillation frequencies are generated at the auxiliary frequency generator 11, the center frequencies f for the respective band groups are generated at the center frequency generator 12 by using the reference frequency and the auxiliary frequencies, and three local oscillation frequencies (f−528 MHz, f and f+528 MHz) for the channels in each band group are generated at the local oscillation frequency generator 13 by up-converting and down-converting each of the center frequencies f by 528 MHz.
The above-described conventional apparatus may generate local oscillation frequencies not interfering with an adjacent channel as long as the respective components operate in an ideal condition. However, the single side band mixer or the dividing circuit actually employed may not always operate in an ideal condition, which leaves possibility for interference with an adjacent channel. In particular, in the case of MB-OFDM system in which the frequency intervals among the adjacent channels are all identical (e.g., 528 MHz), the non-ideal characteristics of the single side band mixer may be a problem. For instance, when the single side band mixers 131a and 131b operate in an ideal condition in the local oscillation frequency generator 13, a clean single frequency component will be outputted from the multiplexer 132 of the local oscillation frequency generator 13. However, if the single side bandmixers 131a and 131b do not operate in an ideal condition, a desired signal of f+528 MHz as well as a side band frequency of f−528 MHz are outputted from the single side band mixer 131a for up-converting the center frequency by 528 MHz. In the same fashion, a desired signal of f−528 MHz as well as a component of f+528 MHz are outputted from the single side band mixer 131b for down-converting the center frequency f. Due to these side band frequencies, interference may occur with another adjacent ultra wide band communication device. That is, because of the side band component of f−528 MHz outputted from the single side band mixer operating in a non-ideal condition, the ultra wide band communication device using the local oscillation frequency of f+528 MHz may interfere with another adjacent ultra wide band communication device using a local oscillation frequency of f−528 MHz.